The invention relates to a method for separating non-ferrous particles of different electrical conductivity, in particular of waste materials, and to a device for carrying out the method.
When separating useful materials, in particular waste materials, it is possible to separate ferromagnetic materials, i.e. in particular iron, without any problems by means of simple magnetic methods. Because of their different electrical conductivity, non-ferrous metals can be further separated from one another and from plastics materials following the removal of the ferromagnetic materials by means of eddy-current separation. In the eddy-current separator a current is induced and thus a force produced in an inducing magnetic field in the particles to be separated by the latter, irrespective of their conductivity, and this current and force expel the particles from the magnetic field. The deflection of non-ferrous metals in the eddy-current separator is in this case determined by the electrical conductivity .sigma. and the density .rho. (relative density) of the materials to be separated. With the density the same, increasing conductivity is accompanied by increasing eddy currents and a corresponding increase in the force that expels the particles from the inducing magnetic field. The force that is to be applied for the same quantitative effect increases with the density. .sigma./.rho. is therefore a suitable characteristic quantity for the qualitative assessment of the separating capacity.
Eddy-current separators of this kind are known in various configurations. For example, EP 0 305 881 A1 describes a method and a device for sorting non-ferrous metal particles by means of eddy-current separation. A conveyor belt runs around a rotating magnet system and the different particles are thrown off in different trajectory parabolas and can thus be sorted to a certain degree. As an improved version, EP 0 339 195 B1 describes a magnetic separator with a conveyor belt, which is guided over a belt drum consisting of non-electrically conductive material, to transport a fraction of particles of greater or lesser electrical conductivity which is to be sorted, with a magnet system which is driven so as to rotate in the belt drum at a higher rotational speed than that of the belt drum, and with a collecting vessel disposed in the material discharge zone of the belt drum for the separated electrically conductive particles. This publication indicates in particular how damage to the belt drum due to particles, in particular iron particles, coming between the conveyor belt and the belt drum can be prevented. This is achieved by a certain geometrical structure.
However the disadvantage of the known eddy-current separators lies in the fact that different non-ferrous metals can only be separated from one another with difficulty and subject to error. This is due in particular to the fact that the physical properties determining the separating capacity only differ slightly.
The object is therefore to improve the separation of non-ferrous metals from one another when using eddy-current separation.